12.6.
Selection Action Buttons

When the user selects a model element in a UML diagram,
several handles are drawn on it to indicate that it is selected
and to provide user interface affordances to resize the node.
ArgoUML also displays some “selection-action
buttons” around the selected model element. See the figure
below for some examples of the handles and
“selection-action buttons”. The two figures for a
class differ because for creating the second one, the shift key
has been depressed.

Figure 12.6.
Some examples of “Selection Action Buttons”.

Selection-action buttons offer common operations on the
selected object. For example, a class node has a button at
12-o'clock for adding a superclass, one at 6-o'clock
for adding a subclass, and buttons at 3-o'clock and
9-o'clock for adding associations. These buttons support a
"click or drag" interaction: a single click creates a
new related class at a default position relative to the
original class and creates a generalization or association; a
drag from the button to an existing class creates only the
generalization or association; and, a drag to an empty space in
the diagram creates a new class at the mouse position and the
generalization or association. ArgoUML provides some automated
layout support so that clicking the subclass button will
position the new classes so that they do not overlap.

Selection-action buttons are transparent. They have a
visibly recognizable rectangular shape and size and they
contain an icon that is the same as the icon used for the
corresponding type of design element on the standard toolbar.
However, these icons are unfilled line drawings with many
transparent pixels. This allows selection-action buttons to be
overlaid onto the drawing area without overly obscuring the
diagram itself. Also, the buttons are only drawn when the mouse
is over the selected model element; if any part of the diagram is
obscured, the mouse can simply be moved away to get a clearer
view of the diagram.